The present invention relates to a phased-array antenna for both ground and airborne satellite communications. More particularly, the invention relates to such an array of antenna elements which are disposed in a pentagonal configuration.
Modem aircraft travel through great distances, often over open water and far from the reach of most conventional communications services. Satellite communications have became crucial to pilots when navigating an aircraft, receiving weather reports and air traffic control information, as well as communicating status and emergency messages. Satellite communication systems located on aircraft provides a means for communication while the aircraft is airborne or on the ground. In addition, satellite communication systems are useful in providing such services as telephone communication, Internet services, and other forms of data exchange to the aircraft passengers. In order to provide these additional services to the aircraft passenger, the satellite antenna system must receive these services through receipt of sufficient amounts of data per unit of time.
The amount of data per unit of time, hereinafter data rate, that the satellite antenna system may support increases as the effective aperture area of the satellite antenna increases. Essentially, the signals originating from the satellite may be received at a higher data rate when the effective aperture area is increased. For aircraft-satellite communication purposes, one requirement is that the aperture efficiency of the aircraft antenna must be suitable to receive a high data rate to provide the communication services required. The aperture efficiency may be defined as the ratio between the physical dimensions of the antenna array and the effective aperture area of the antenna array. A high aperture efficiency implies that this ratio is close to unity.
The aperture efficiency is an important design consideration for satellite antennas placed in the tail section of modern aircraft. The tail sections of modern aircraft tend to be very narrow and, consequently, there is a limit to the size of antenna that can be placed in this tail section. In, order to compensate for the size limitation, the aperture efficiency of the antenna must be increased. Another design consideration is that, typically, the antenna will be mechanically steered within the tail of the aircraft in order to scan the coverage required. This additional movement demands that the diameter of the antenna be small enough so that the antenna fits within a radome covering the top portion of the aircraft tail section.
One solution to the above requirements is the use of mechanically-steered phased-array antennas. Phased-array antennas have been markedly suitable for aircraft purposes. These antennas can, in some cases, have a high aperture efficiency and permit the antenna beam to be directed at various satellites regardless of aircraft orientation. The mechanically-steered phased-array antenna consists of a group of antenna elements that are distributed and oriented on a planar surface in various spatial configurations. The amplitude and phase excitation of each antenna element is fixed since the beam is pointed mechanically.
U.S. Pat. No. 4,123,759, issued to Hines et al., discloses a phased-array antenna where at least three radiating elements are located at the corners of regular polygon. According to Hines, the preferred embodiment for radio communications is a square-array. The crux of Hines teaching is in maximizing the radiation in one of four primary directions to achieve omni-directional coverage around the horizon. However, one shortcoming of Hines is that the preferred polygon configuration is not ideal for satellite antenna applications where aperture efficiency is critical for providing communication services. Furthermore, the antenna elements described by Hines have a very broad beam width thus limiting the antenna""s gain. The broad beam width, however, is a hindrance in satellite applications where the maximum gain possible is required.
U.S. Pat. No. 6,115,005, issued to Goldstein et al., teaches a helical antenna arrangement including a large number, 36 in the preferred embodiment, of helical antenna elements with axes parallel to the antenna boresight axis. The antenna elements have a spatially aperiodic distribution. The mutual spacing between any two antenna elements of the array is at least twice the wavelength of the operating frequency of the antenna. The antenna elements are converted to a signal distribution unit which contains a signal network through which the antenna beam or radiation pattern is controlled.
The present invention provides an antenna array of helical elements having a pentagonal configuration with exceptionally high aperture efficiency and low swept volume for a given gain. The antenna elements belonging to the antenna array should have very narrow beams and be fixed in position and connected to a phasing/combining network. Furthermore, the present invention provides an antenna array suitable for mounting in the tail section of an aircraft for satellite communications.
The present invention provides a pentagonal antenna array having a high aperture efficiency and a suitably high overall gain and low antenna noise temperature. The high aperture efficiency of this antenna system provides an overall system capacity suitable for broadband communication services. The antenna array consists of five antenna elements each located at a separate vertex of a pentagon. The antenna elements are helical antennas to provide a narrow antenna beam width. The antenna array itself is supported on a base platter which may be steered to point at a satellite using conventional gimbal ring apparatus. The base platter is a planar reflector to reflect the antenna element radiation in the rear direction and thereby reduce the antenna backlobe levels. The input power and the signal transmitted are fed through a phasing/combining network. The phasing/combining network appropriately divides the signal and the input power and phases the signal, prior to feeding the signal to each of the five antenna elements.
The antenna elements are positioned at the points of a pentagon, which are 72xc2x0 apart, around the antenna base. Adjacent elements have a 72xc2x0 relative angular rotation and a compensating 72xc2x0 relative excitation phase in order to ensure that the elements are all radiating in phase, which results in very uniform excitation of the entire volume of the antenna and the elimination of polarisation loss. Each helical antenna element launches an axial wave that is circularly polarised. The individual helices are angularly displaced 72xc2x0 and the phase of the current provided to the windings is sequentially offset by 72xc2x0 such that the radiated fields are in phase. This sequential xe2x80x9cdisplacementxe2x80x9d virtually eliminated polarisation loss by avoiding the alignment of the principal axes of the individual polarisation ellipses. The individual helices positioning is arranged that each one has a direction 72xc2x0 from the position of its neighbours. If they were fed in phase their radiated signals would be out of phase. But by feeding them 72xc2x0 from their neighbours their radiated signals are in phase.
In a first aspect, the present invention provides an antenna system comprising:
a base;
five helical antenna elements mounted on said base, each antenna element having a base end and a terminal end and being located at a separate vertex of a pentagon, and each element further having an antenna element feed connected at the base end thereof; and
a phasing/combining network for feeding input power to each antenna element feed;
wherein each of the elements is supported by the base at the base end of each antenna element.
In a second aspect, the present invention provides an antenna system comprising:
a base having a planar surface;
five helical antenna elements mounted on said base with their helix axes orthogonal to the plane surface of the base, each of the helical antenna elements being located at a separate vertex of a pentagon, and each element having an antenna element feed located at a base end of the element; and
a phasing/combining network for feeding input power to each antenna element.
In a third aspect, the present invention provides an antenna system including:
a base having a planar surface;
five multi-filar helical antenna elements mounted on said base with their helix axes orthogonal to the planar surface of the base, each of the elements being located at a separate vertex of a pentagon, each of the five multi-filar helical antenna elements having a feed network coupled to a base end thereof and each feed network having a feed end located at the base end of a corresponding element; and
a phasing/combining network for connecting the feed network of each element;
wherein each element is supported by the base at the base end of the element, and wherein the phasing/combining network feeds input power to the feed end of each feed network.
In a fourth aspect, the present invention provides an antenna system comprising:
a base having a planar surface;
five helical antenna elements mounted on said base with their helix axis orthogonal to the planar surface of the base, each of the five helical antenna elements being located at a separate vertex of a pentagon, each of the five helical antenna elements being angularly displaced by 72xc2x0 relative to its two adjacent elements, each of the five helical antenna elements having a feed network coupled to a base end of the helical antenna element, and each feed network having a feed end located at the base end of a corresponding helical antenna element, the feed network exciting the elements with relative phases of 0xc2x0, 72xc2x0, 144xc2x0, 216xc2x0, 288xc2x0 or 0xc2x0, xe2x88x9272xc2x0, xe2x88x92144xc2x0, and xe2x88x92288xc2x0.